Eerie presence, especially if unexplained. Not really here because it can dissipate. It is only a sensation, I assume frustration refers to the frustration of the riddle. After you go somewhere your smell fills the path behind you. Note that this explanation also works for the answer noise assuming you make some noise while walking but smell is what I picked since the word for smell is both the sense of smell and a smell itself (possibly only in slang)

the Past?
The past exists, but not anymore (philosophically speaking), which makes it scary/eerie for some.
Not entirely sure about the third/fourth lines, but I think it has to do with memories, particularly bad ones that cause grief.
You and your past go separate ways - while you go forward in time, the past is always backwards. Also, the past is always in the space between what once was the past and where you are in the present.